VERY dangerous to ride in. And when used to lunge, they work there but do not associate much to undersaddle. The same effect (of forward/down/out/open) can be gotten by proper riding technique to create the same behaviors AND be safe.
I've ridden in chambons at the request of owners of horses that I was riding. Didn't like them, but also did not think they did anything either way. I didn't have problems keeping those particular horses "together" with or without the chambon. The horses were only going in them because our mutual (former) trainer had latched on that that as the gimmick of the moment.
Where am I and what am I doing in this handbasket?
I don't ride in one - I don't like the idea of being on a horse when I can't "release" their head in an emergency (w/draw reins, de gogue, you can release the pressure). That's just what I feel comfortable with.
But I'll modify sarcam's statement:
You should be able to get a better result with proper use of hand and leg but reality often interferes with what should happen. First off, we aren't all great, second, even the great amongst us (including some legends in their own mind) aren't great all the time, and third, not all horses got the memo about hand and leg. Every "gimmick" got there because it actually worked well for somebody, and that somebody wasn't Joe Nobody in the local show fo rit to catch on.
So I'll use a gimmick now and then, but it has to be a temporary bridge, either because of my failure to properly communicate with the horse or the horse just being one of those who didn't get the memo. Let's face it, that applies to all of us and you have to ride the horse you are on, not the one in the book.
I have ridden in one, under the supervision of a trainer when I was young. I had a horse with a whole lot of baggage. And this helped her greatly. And I have used it a few times since in certain cases. However they are extremely dangerous, and you really have to know what you are doing. I have never used them for a horse I have started myself, or for 99% of the horse I have trained.
They can be a great tool but they alone will not make you ride properly. You still need to ride back to front, maybe even more so when using one. And you need to have wonderful hands, that can be soft and steady, you must soften the instant the horse does. And that is a skill many riders do not have, and if you have them, you probably wouldn't need to use one in the first place. My advice is to use proper riding instead, and take the time.
It is a tool, and like any tool it can be good or bad or nothing at all depending on the rider's skill and the horse's issues. I used one quite a bit in the past on a defensive, reactive horse, and often jumped in it. (Gasp!! Call the Gadget Police!)
I wouldn't use one now because I have other tools and skills that I think are more effective, but not everyone does, and I have seen a chambon used effectively by other people. My horses now have never worn a chambon, draw reins, or even martingales- but I don't go around judging other people who use those things. Tools don't produce bad riding- it is there with or without them. Tools *can* exacerbate bad riding, but a chambon isn't going to turn anyone into a serial killer, and bad riding isn't going to get better without external help!
You can take a line and say it isn't straight- but that won't change its shape. Jets to Brazil
(what did they answer? THe simple answer is that they have not been given the riding/training skills to be able to ask the horse a question to which they can answer 'yes', so there is a bandaid repair of symtoms rather than treating causes. mho).